Does it say in the book, I'm pretty sure it doesn't in the films, how Sonny makes his bones and around what age? I can't remember if they say.

If I remember correctly, the novel says something about Sonny Corleone making his bones at age 19.

Sonny was caught for a robbery at 16, when vito found out he lost his temper, but then sonny said he saw his father kill fanucci, that changed everything, vito knew that sonny would now have to join the family business, due to vito's thoughts of "a man has but one destiny" sonny would be the succeser to the don.

As Chris pointed out, it does make referance to Sonny making his bones, but it doesnt say how. what I've always wondered is why would any Mafia boss's son would have to make his bones. before the decline of Omerta, no son would sell out his father. Fredo didnt even sell out his father.

I know sumwhere in the novel it says how he made his bones... something about killing 2 repair guys or something like that. I forgot.

thanks for the info, i was wondering the same.

If I'm not mistaken, Sonny made his bones when he took over his own regime after the Don was shot the first time (by a couple of Irish guys, I think). The book says how great he was in urban warfare (it compares him to Napoleon) and how he was feared by everybody because he was completely ruthless in warfare, something Don Corleone was not.

The book doesn't say exactly how Sonny made his bones, but it does say how he took over the family's defense after his father was shot, and showed "a genius for urban guerilla warfare."
The episode of the two furnace repair guys is a gem of a story, but Sonny didn't kill them.

yeah Sonny didn't make his bones when confronting the repair men, that was a test of leadership by his father, as he already made his bones by managing to wage war while the Don was shot.

Cool, thanks guys.

In a semi-related bit, In GF III, Vincent makes a reference to Sonny being "The prince of the city".

I was wondering why he would refer to him this way. Obviously, what he heard about Sonny was from his mother (the bridesmaid who got nailed against the door in GF I), because Sonny was dead before Vincent was even born, or at the very least had to have died when Vincent was very young.

Probably showing filial pride (and at the same time reminding everyone of his noble parentage).
As you're probably aware, Vincent is purely a creature of GFIII. There was no mention in the novel, or the earlier two films, that Sonny impregnated Lucy.